1. Field of the Invention
The present invention relates to a switching mode power supply (SMPS). More specifically, the present invention relates to an SMPS for generating a bias voltage of a pulse width modulator (PWM) controller.
2. Description of the Related Art
An SMPS converts a DC voltage into at least one other DC voltage. In some systems the DC output voltage is controlled to be greater or lesser than the input voltage. The SMPS is applied to power electronic devices, and in particular, battery power supply devices such as mobile phones and laptop computers.
It is required to supply a bias voltage Vcc to a PWM controller to control the operation of a main switch in order to operate an SMPS in the initial stage of driving the SMPS. A general method for supplying a bias voltage to the PWM controller is to supply the bias voltage Vcc by the SMPS in the initial driving stage of the SMPS. After the initial driving stage the main switch begins to operate. This makes it possible to supply the bias voltage from the secondary coil of a transformer. A terminal for supplying the bias voltage (bias voltage terminal) and a terminal coupled to the main switch (main switch terminal) are high voltage terminals. Reducing the number of high voltage terminals advantageously simplifies the system design.
U.S. Pat. No. 5,014,178, incorporated herein by reference in its entirety, discloses a conventional system and method for unifying the above bias voltage terminal and main switch terminal. The system of the patent (i) supplies the bias voltage until the voltage at the bias voltage terminal reaches a first reference voltage in the initial driving stage; (ii) in addition supplies the same voltage by using the secondary coil of the transformer during the period when the voltage rises from the first reference voltage to a second reference voltage; and (iii) supplies the bias voltage by using the secondary coil of the transformer alone when the bias voltage exceeds the second reference voltage. However, this conventional system has a complicated design since it uses two reference voltages to supply the bias voltage. Also, in the conventional system information on the output voltage in fed back at the same terminal where the bias voltage is supplied. The conventional method provides information on the output voltage through the secondary coil of the transformer indirectly, and hence the output voltage may not be conveniently regulated.